diff --git a/src/backend/commands/tablespace.c b/src/backend/commands/tablespace.c
index 427d896f619..6e8f6a74d89 100644
--- a/src/backend/commands/tablespace.c
+++ b/src/backend/commands/tablespace.c
@@ -1153,6 +1153,7 @@ GetDefaultTablespace(char relpersistence)
 
 typedef struct
 {
+	/* Array of OIDs to be passed to SetTempTablespaces() */
 	int			numSpcs;
 	Oid			tblSpcs[FLEXIBLE_ARRAY_MEMBER];
 } temp_tablespaces_extra;
@@ -1202,6 +1203,7 @@ check_temp_tablespaces(char **newval, void **extra, GucSource source)
 			/* Allow an empty string (signifying database default) */
 			if (curname[0] == '\0')
 			{
+				/* InvalidOid signifies database's default tablespace */
 				tblSpcs[numSpcs++] = InvalidOid;
 				continue;
 			}
@@ -1228,6 +1230,7 @@ check_temp_tablespaces(char **newval, void **extra, GucSource source)
 			 */
 			if (curoid == MyDatabaseTableSpace)
 			{
+				/* InvalidOid signifies database's default tablespace */
 				tblSpcs[numSpcs++] = InvalidOid;
 				continue;
 			}
@@ -1338,6 +1341,7 @@ PrepareTempTablespaces(void)
 		/* Allow an empty string (signifying database default) */
 		if (curname[0] == '\0')
 		{
+			/* InvalidOid signifies database's default tablespace */
 			tblSpcs[numSpcs++] = InvalidOid;
 			continue;
 		}
@@ -1356,7 +1360,8 @@ PrepareTempTablespaces(void)
 		 */
 		if (curoid == MyDatabaseTableSpace)
 		{
-			tblSpcs[numSpcs++] = curoid;
+			/* InvalidOid signifies database's default tablespace */
+			tblSpcs[numSpcs++] = InvalidOid;
 			continue;
 		}
 
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 6d016a79e93..03d43fdada1 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -253,8 +253,10 @@ static AllocateDesc *allocatedDescs = NULL;
 static long tempFileCounter = 0;
 
 /*
- * Array of OIDs of temp tablespaces.  When numTempTableSpaces is -1,
- * this has not been set in the current transaction.
+ * Array of OIDs of temp tablespaces.  (Some entries may be InvalidOid,
+ * indicating that the current database's default tablespace should be used.)
+ * When numTempTableSpaces is -1, this has not been set in the current
+ * transaction.
  */
 static Oid *tempTableSpaces = NULL;
 static int	numTempTableSpaces = -1;
@@ -2518,6 +2520,9 @@ closeAllVfds(void)
  * unless this function is called again before then.  It is caller's
  * responsibility that the passed-in array has adequate lifespan (typically
  * it'd be allocated in TopTransactionContext).
+ *
+ * Some entries of the array may be InvalidOid, indicating that the current
+ * database's default tablespace should be used.
  */
 void
 SetTempTablespaces(Oid *tableSpaces, int numSpaces)